Covering for airfoil surfaces



June 22, 1943. w. HARPER, JR

COVERING FOR AIRFOIL SURFACES Filed Feb. 13, 1940 ,Illl lll'.

INVENTOR Wzz/A/y 4/45/29?) (/5? ATTORNEY Patented June 22, 1943 UNITED STATES PATENT OFFICE 3 Claims.

This invention relates to a covering for an airfoil surface, and aims to reduce skin friction and turbulence.

As a result of experiments and investigation I have discovered that the so-called skin friction of an airfoil surface, which is really the effect of turbulence between the boundary layer and the air beyond, may be reduced by providing the airfoil surface with a nap whose filaments may be bent down in the direction of air flow along the surface.

My invention is applicable to all the surfaces of an airplane along which the air flows and is of especial value on the wing surfaces. It may also be applied to the surfaces of the propeller blades. It is of especial value in airplanes designed for very high speeds, and is, so far as I am aware, the only type of airfoil surface which will avoid turbulence at speeds above six-tenths of the velocity of sound.

An airfoil covering embodying my invention consists of a tegument provided with a nap of closely spaced, normally upstanding, fine filaments which are flexible and slightly resilient so that, when the airfoil surface is in motion, the filaments of the nap are bent down by the air flow along the airfoil surface in the direction of the air flow. In this position, they effectively prevent turbulence at high speeds.

The covering may consist of an ordinary fabric provided with a nap, such as velvet.

An important feature of my invention consists in making the filaments of the nap of glass so that they are durable and not affected by moisture. In order to provide the requisite flexibility, I use glass filaments of less than 0.004 inch in diameter. This preferred form is illustrated in the accompanyiny drawing in which Fig. 1 is a cross-section of an airfoil surface showing the position of the nap when the airfoil is stationary, and Fig. 2 is a similar view showing the position of the nap when the airfoil surface is moving rapidly through th air.

In the drawing, A represents glass fibres of less than 0.004 inch in diameter and preferably about inch in length. B represents a tegument in which the inner ends of the filaments are held.

As appears from Fig. 1, the glass filaments A of the nap when in their normal or upstanding position when the airfoil is stationary are inclined rearwardly at an angle of about 45 to the airfoil surface. When the airfoil is moving rapid- 1y through the air, the filaments of the forward part of the surface are bent down more nearly in line with the surface while those at the rear part of the surface remain substantially in their normal position, both the depressed and upstanding fibres being inclined in the direction of the air flow along the surface.

What I claim is:

l. A covering for an airfoil surface, comprising a nap of flexible glass filaments normally inclined outwardly relative to the airfoil surface in the direction of the airflow along said surface.

2. A covering for an airfoil surface, comprising a nap of glass filaments less than 0.004 inch in diameter normally inclined outwardly relative to the airfoil surface in the direction of the airfiow along said surface.

3. A covering for an airfoil surface, comprising a tegument and a multiplicity of fibres set in said tegument at an acute angle so that they extend upwardly therefrom in the direction of air flow along the surface, the fibres being sufficiently flexible to be bent down in the direction of air flow by flow of air along the surface.

WILLIAM HARPER, J R. 

